In recent years, there has been a tendency towards power systems having more and smaller energy sources providing input to the power networks. The focus on climate change and the consequential focus on reduction of CO2 emissions lead away from large coal fired power generators providing a significant share of the total input to the power system, and towards power systems where the share of power from renewable energy sources, such as power from wind, water or solar energy sources, is significantly higher than hitherto. However, renewable energy sources are relatively uncontrollable and typically each renewable energy source is relatively small and they are typically spread over a wide area in the power system.
The existing transmission systems are not necessarily designed to handle these new production patterns, and traditional approaches where security assessment has been carried out off-line by system planners are insufficient in today's complex networks, which was clearly seen from the major blackouts in electric power systems in Sweden and Denmark in September, 2003 and in North-Eastern and Mid-Western United States and parts of Canada in August 2003, each affecting millions of people.
In response to these new production patterns, sophisticated computer tools have been developed for power system analysis and led e.g. to the use of Phasor Measurement Units (PMU's) that provide synchronized measurements in real time, of voltage and current phasors along with frequency measurements. The introduction of PMUs together with advances in computational facilities and communications, opened up for new tools for controlling, protecting, detecting and monitoring of the power systems. However, even though these tools are capable of determining whether a power system is in a stable or an unstable condition, the tools have not been efficient in determining the stability boundaries of the power system or the system security margins.
Some systems have been suggested using only the system voltage phase angle measurements for assessing the system operating conditions. However, it is a disadvantage of these systems that a representation using only the voltage phase angle measurements does not provide a unique representation of a power system operating condition.
Furthermore, multidimensional nomograms have been suggested for the purpose of monitoring the overall system stability or security boundaries, however, the critical boundaries are determined in an offline analysis where multiple critical boundary points have been determined by stressing the system in various directions away from a given base operating point. However, it is a disadvantage of this approach that the boundaries are determined for a specific base case and a fixed system topology. If the system is subjected to any topological change (e.g. tripped lines due to maintenance), the actual approach may introduce an uncertainty for the assessment of security margin, as it has been based on the non-changed topological structure.
On-line monitoring of maximum permissible loading of a power system has also been suggested (M. H. Hague, “On-line monitoring of maximum permissible loading of a power system within voltage stability limits”, IEE Proceedings, Generation, Transmission and Distribution, vol. 150, no. 1, 20 Nov. 2002, pages 107-112), wherein locally measurable quantities, such as bus voltage magnitude and the active and reactive components of load power are measured and a maximum permissible loading and voltage stability margin of the power system is estimated at a node in the system. By calculating a maximum loading of a system, transformers, such as e.g. transformer stations, correlated with the node may have to shed load or to adjust e.g. the turns ratio to ensure that the maximum loading of the system is not exceeded. The maximum load point at a node is reached when the Thevenin impedance of the system as seen from the node, equals the load impedance seen from the same node. Thus, this method provides an estimate of the value of maximum power that can be received at the node of concern.
The voltage stability mechanisms are related to the capability of transformers with adjustable winding ratio to maintain constant voltage levels at one of the nodes to which they are connected. If the point of maximum power that can be transmitted to the node is reached, the control actions of the transformers have a destabilizing effect on the voltages, resulting in a very slow uncontrollable decrease in voltage magnitude.
This instability mechanism is caused by the control actions of the adjustable transformers, and the instability process can take several minutes to unfold.